Car Talk: Bad in­jec­tors could be cause of en­gine’s stum­bling

Dear Car Talk: I’ve never had any prob­lems with my 2012 Hyundai Elantra, un­til now. It has a 1.8-liter en­gine, au­to­matic trans­mis­sion and 156,000 miles. At speeds be­tween 35 and 45 mph, it some­times will hes­i­tate and clunk. I in­stalled new spark plugs and had a mo­tor mount re­placed. Also, the idle is run­ning re­ally high. And there’s a growl un­der the hood.

My mileage has gone from 33.5 mpg to un­der 20. I changed out the cat­alytic con­verter, re­placed the ser­pen­tine belt, and cleaned the throt­tle body. Noth­ing has helped. Is the tim­ing too rich? The check en­gine light is not on, and I don’t have any fault codes stored in the com­puter. Any ideas? – Bill Dear Bill: This sounds like a real sweet­heart, Bill. The most in­ter­est­ing clue you gave me is that the idle is run­ning high.

There’s ac­tu­ally no way to ad­just the idle on mod­ern cars, like you could by turn­ing a screw on your fa­ther’s Chevy. It’s all com­puter-con­trolled now. So if the idle is con­sis­tently high, that means some­thing’s wrong. The fact that the check en­gine light has not come on tells me it’s not likely to be an en­gine sen­sor, or any­thing in the com­put­er­ized en­gine man­age­ment sys­tem. So it’s got to be some­thing that’s fool­ing the com­puter into think­ing you’re step­ping on the gas.

And while it could be a vac­uum leak, my first guess would be a bad in­jec­tor or two. Or four. If one or more of your in­jec­tors were send­ing too much ga­so­line into the cylin­ders, that could ex­plain every­thing. With ex­tra ga­so­line in the mix­ture, the down­stream oxy­gen sen­sor would tell the com­puter to send in more air, to pro­tect the cat­alytic con­verter from be­ing dam­aged. That would boost up your idle, lower your mileage and – wait for it – cause the en­gine to stum­ble or hes­i­tate some­times.

When a cylin­der is flooded with too much fuel, that ex­tra fuel can ex­tin­guish the spark. That cre­ates an en­gine mis­fire, and causes that hes­i­ta­tion you feel. So you’ll need to take this car into a me­chanic and ask him to look at your in­jec­tors. As for the growl­ing noise, I’d look for a miss­ing neigh­bor­hood dog, Bill. What­ever’s caus­ing the growl­ing (and it could be some­thing like a bad wa­ter pump, idler pul­ley or al­ter­na­tor), I sus­pect it’s un­re­lated to the other is­sues.

Re­plac­ing in­jec­tors is not go­ing to be cheap. But nei­ther is fill­ing up twice as often, and join­ing the Cat­alytic Con­verter of the Month club. So fig­ure it out, and get it fixed.